Insofar as fiber optic sensors are often interferometric in design, in which case they typically require modulation of optical phase in one or more arms in order to bias the interferometer at a point of maximal sensitivity, for example, or to facilitate homodyne or other synchronous detection, phase modulators of various sorts are employed to achieve the requisite modulation. In current practice, phase modulation is often achieved in a discrete integrated optic (IO) component, often a lithium-niobate (LiNbO3) chip.
Optical fiber sensors are reviewed in Lee, Optical Fiber Technology, Elsevier (2003), which is incorporated herein by reference. Fiber-optic gyroscopes (FOG), in particular, have evolved into instruments that are currently useful, robust and capable of imposing state-of-the-art experimental constraints on physical theories, as reviewed by Lefèvre, The Fiber-Optical Gyroscope (2d ed.), Artech House (2014), which is also incorporated herein by reference.
The use of poling to induce second-order optical nonlinearities in optical fibers, for both silicate and chalcogenide glasses, has been discussed in the literature, as by Fleming, et al., “Poled Glass and Poled Fibre Devices,” J. Ceramic Soc. Japan, pp. 1007-23, (2008) and Liu et al., “Second harmonic generation in Ge20As25S55 Glass Irradiated by an Electron Beam,” Opt. Lett., vol. 26, pp. 1347-49 (2001), both of which papers are incorporated herein by reference. The poling mechanism entails creating a quasi-permanent charge distribution in the glass by cooling it under a strong applied electric field.
To the best of the knowledge of the current inventors, the use of poled optical fibers for sensing applications has been limited to date to sensing electric fields. U.S. Pat. No. 6,385,377 (to Brueck) provides an example of that application.
Until the current invention, it had always been thought that modulation of the phase of light in an arm of a fiber sensor requires either that light be coupled out of the fiber into a discrete modulation component or else that the fiber would need to be deformed, by means of a piezo-electric phase modulator, or otherwise, with the attendant induction of mode coupling. Both of the foregoing methods are subject to drift and non-linear response. Proton-exchanged lithium-niobate integrated optics phase modulators, often employed as phase modulation elements in fiber optic sensors, are heavily influenced by charge migration phenomena by virtue of their ferroelectric characteristics. A comprehensive review of the performance and limiting characteristics of LiNbO3 modulators may be found in Wooten, “A Review of Lithium Niobate Modulators for Fiber-Optic Communications Systems,” IEEE J. Sel. Top. in Quantum Electronics, vol. 6, pp. 69-82 (2000), which is incorporated herein by reference. Charge migration phenomena in LiNbO3 result in drift and nonlinear response at low frequencies and environmental sensitivity, features that ultimately limit the performance of inertial measurement systems based on fiber optic gyroscopes (FOGs).
It would be desirable to achieve higher frequency modulation and imperviousness to drift and nonlinear response inherent in existing modulation technologies. Methods for doing so are taught in accordance with embodiments of the present invention, as described below.